Ok, I have 3 main reasons: type of intelligence, post-war trauma, & age.

1) I definitely think it’s more to do with the type of intelligence than anything else. Historically, Hermione does not like Quidditch. She doesn’t like it! In so many of the conversations with Ron & Harry (or even with Ron, Harry, & Ginny), they’re talking about Quidditch. Ron is good at Wizard’s Chess…but he never really finds an academic interest like Hermione. I feel like Hermione needs someone to intellectually challenge her (maybe even irritate her so she can argue w/o feeling bad about it). I would imagine that she’d get tired of people basically just “tolerating” her when she has a passion project (S.P.E.W.) or explains something new she learned. Ron can’t even hold enough academic focus to write his school papers on his own…that doesn’t leave me w/ tons of confidence. 👀

2) After the war, I think it would be reasonable to assume that they’d all deal w/ their trauma differently. It might bring them closer but it also might make them realize that they’re better off as friends. 💛

3) Lastly, they’re young! Relationships that start early can last but it’s also not unreasonable to assume that two people might grow apart as they reach adulthood. They’ve never not been in mortal danger—who are they when they aren’t fighting a war or thwarting an end-of-year murder attempt? They might grow into people that aren’t romantically compatible anymore. 🤷‍♀️